132 ARBOREAL MAN 



speak of a phylogenetic flattening of the chest, we must 

 also be prepared to admit that the narrow quadrupedal 

 type of chest is itself a modification from the presumably 

 flatter chest that was the possession of the first mammalian 

 forms. 



The change from the narrow pointed chest to the broad 

 flat chest is, for the most part, effected by the falling 

 backwards, towards the backbone, of the whole chest, 

 as the animal becomes more adapted to maintaining its 

 body axis upright. The ribs of a quadrupedal animal 

 tend to dispose themselves as oval hoops, hung in their 

 long axis from the backbone; the ribs of an animal with 

 an upright axis tend to dispose themselves as rounded 

 hoops, surrounding the backbone as hooj)S surround a 

 pole. 



In this way the backbone tends to become, not the 

 ridge from which the hoops are hung, but a central prop 

 within the circle of the hoops; to do this the backbone 

 has to sink into the back of the series of hoops, pushing 

 them in before it as it goes. This simple mechanical 

 alteration effects a double change. In the quadrupedal 

 animal the breastbone projects as a keel; the backbone 

 projects as a ridge. In the animal with the upright axis 

 the breastbone is merely part of the evenly rounded front 

 of the chest, the backbone merely a part of the evenly 

 rounded back. In Man the process culminates in a chest 

 which is flat in front, and a back which is flat behind. 

 This is a simple mechanical process ; in no sense can it be 

 said to be due to the assumption of the upright jDosture, 

 if by that posture the erect walking posture is meant. 

 It did not come suddenly into the possession of the human 

 stock when that stock took to walking with the soles of 

 the feet planted flat upon the level surface of the earth; 

 it was already in being when, in life among the branches, 

 the animal squatted or hung with its body erect. This 

 is a change of bodily conformation, and, as such, needs 

 treatment elsewhere; it does not so directlv concern the 



